Obesity, defined as an excess of body fat relative to lean body mass, is associated with important psychological and medical morbidities, the latter including hypertension, elevated blood lipids, and Type II or non-insulin-dependent diabetes melitis (NIDDM). There are 6–10 million individuals with NIDDM in the U.S., including 18% of the population of 65 years of age [Harris et al., Int. J. Obes., 11:275–283 (1987)]. Approximately 45% of males and 70% of females with NIDDM are obese, and their diabetes is substantially improved or eliminated by weight reduction [Harris, Diabetes Care, 14(3):639–648 (1991)]. As described below, both obesity and NIDDM are strongly heritable.
The assimilation, storage, and utilization of nutrient energy constitute a complex homeostatic system central to survival of metazoa. Among land-dwelling mammals, storage in adipose tissue of large quantities of metabolic fuel as triglycerides is crucial for surviving periods of food deprivation. The need to maintain a fixed level of energy stores without continual alterations in the size and shape of the organism requires the achievement of a balance between energy intake and expenditure.
An individual's level of adiposity is, to a large extent, genetically determined. Examination of the concordance rates of body weight and adiposity amongst mono- and dizygous twins or adoptees and their biological parents have suggested that the heritability of obesity (0.4–0.8) exceeds that of many other traits commonly thought to have a substantial genetic component, such as schizophrenia, alcoholism, and atherosclerosis [Stunkard et al., N. Engl. J. Med., 322:1483–1487 (1990)]. Familial similarities in rates of energy expenditure have also been reported [Bogardus et al., Diabetes, 35:1–5 (1986)]. Genetic analysis in geographically delimited populations has suggested that a relatively small number of genes may account for the 30–50% of variance in body composition [Moll et al., Am. J. Hum. Genet., 49:1243–1255 (1991)].
Rodent models of obesity include seven apparently single-gene mutations. The most intensively studied mouse obesity mutations are the ob (obese) and db (diabetes) genes. When present on the same genetic strain background, ob and db result in indistinguishable metabolic and behavioral phenotypes, suggesting that these genes may function in the same physiologic pathway [Coleman et al., Diabetologia, 14:141–148 (1978)]. Mice homozygous for either mutation are hyperphagic and hypometabolic, leading to an obese phenotype that is notable at one month of age. The weight of these animals tends to stabilize at 60–70 g (compared with 30–35 g in control mice). ob and db animals manifest a myriad of other hormonal and metabolic changes that had made it difficult to identify the primary defect attributable to the mutation [Bray et al., Am. J. Clin. Nutr., 50:891–902 (1989)]. As noted below, identification of the OB gene led to an understanding of one molecular element.
Each of the rodent obesity models is accompanied by alterations in carbohydrate metabolism resembling those in Type II diabetes in man. In some cases, the severity of the diabetes depends in part on the background mouse strain [Leiter, Endocrinology, 124:912–922 (1989)]. For both ob and db, congenic C57BL/Ks mice develop a severe diabetes with ultimate β cell necrosis and islet atrophy, resulting in a relative insulinopenia. Conversely, congenic C57BL/6J ob and db mice develop a transient insulin-resistant diabetes that is eventually compensated by β cell hypertrophy resembling human Type II diabetes.
The phenotype of ob and db mice resembles human obesity in ways other than the development of diabetes—the mutant mice eat more and expend less energy than do lean controls (as do obese humans). This phenotype is also quite similar to that seen in animals with lesions of the ventromedial hypothalamus, which suggests that both mutations may interfere with the ability to properly integrate or respond to nutritional information within the central nervous system. Support for this hypothesis comes from the results of parabiosis experiments [Coleman, Diabetologia, 9:294–298 (1973)] that suggest ob mice are deficient in a circulating satiety factor and that db mice are resistant to the effects of the ob factor (possibly due to an ob receptor defect). These experiments have led to the conclusion that obesity in these mutant mice may result from different defects in an afferent loop and/or integrative center of the postulated feedback mechanism that controls body composition.
Using molecular and classical genetic markers, the ob and db genes have been mapped to proximal chromosome 6 and midchromosome 4, respectively [Bahary et al., Proc. Nat. Acad. Sci. USA, 87:8642–8646 (1990); Friedman et al., Genomics, 11:1054–1062 (1991)]. In both cases, the mutations map to regions of the mouse genome that are syntenic with human, suggesting that, if there are human homologs of ob and db, they are likely to map, respectively, to human chromosomes 7q and 1p. Defects in the db gene may result in obesity in other mammalian species: in genetic crosses between Zucker fa/fa rats and Brown Norway +/+rats, the fa mutation (rat chromosome 5) is flanked by the same loci that flank db in mouse [Truett et al., Proc. Natl. Acad. Sci. USA, 88:7806–7809 (1991)].
A major advance in understanding the molecular basis for obesity occured with the cloning of the ob gene. The mouse obesity (ob) gene encodes an adipose tissue-derived signaling factor for body weight homeostatis [Zhang et al., Nature, 372:425 (1994); U.S. patent application Ser. No. 08/292,345 filed Aug. 17, 1994; U.S. patent application Ser. No. 08/483,211, filed Jun. 7, 1995, each of which is hereby incorporated by reference in its entirety]. Several recent studies have shown that recombinant OB protein (leptin) purified from Escherichia coli can correct the obesity related phenotypes in ob/ob mice when exogenously administered [Campfield et al., Science, 269:546 (1995); Pellymounter et al., Science, 269:540, (1995); Halaas et al., Science, 269:543 (1995); Stephens et al., Nature, 377:530 (1995)]. Weight-reducing effects of recombinant leptin were also observed in normal mice and mice with diet-induced obesity. Although the target tissues that mediate the effects of leptin have not yet been defined, the instant inventors have predicted the brain as a target of leptin activity. Indeed, the work of Campfield et al. (supra) and Stephens et al. (supra) demonstrates that leptin introduced into the lateral or third brain ventricle is effective at low doses, arguing for a direct central affect of the leptin molecule.
Recent studies have suggested that obese humans and rodents (other than ob/ob mice) are not defective in their ability to produce leptin mRNA or protein and generally produce higher levels than lean individuals [Maffei et al., Nature Med., 1:1155 (1995); Considine et al., J. Clin. Invest., 95:2986 (1995); Lonnqvist et al., Nature Med., 1:950 (1995); Hamilton et al., Nature Med., 1:953 (1995)]. These data suggest that resistance to normal or elevated levels of leptin may be important factors in human obesity. However, a recent report of identification of a leptin receptor did not identify any mutations in the ob allele [Tartaglia et al., Cell, 83:1263–1271 (1995)].
Accordingly, there is a need in the art to identify a receptor for leptin.
There is a further need to characterize mutations in the leptin receptor, particularly as they may be associated with obesity.
There is a still further need to identify and characterize functions of the leptin receptor, or variants thereof.
These and other needs in the art are addressed by the present invention.
The citation of any reference herein should not be construed as an admission that such a reference is available as prior art to the application.